This invention relates to a precision placement apparatus comprising a dual prism assembly for providing superposed images of facing surfaces. More particularly, this invention relates to a dual prism probe for such apparatus that includes a liquid crystal shutter that is selectively actuated for distinguishing features during image analysis.
In the manufacture of microelectronic packages, assembly of a component to a substrate may require precise alignment of features on facing surfaces. For example, in forming solder bump interconnections, solder bumps affixed to an integrated circuit die are aligned with bond pads disposed on a printed circuit board or like substrate. For assembly, the component and the substrate are mounted onto moveable fixtures so that the component is spaced above the substrate. A dual prism assembly is positioned between the component and the substrate with one face toward the component for receiving an image of the component surface and an opposite face toward the substrate for receiving an image of the substrate surface. These images are fed to a camera, which, in turn, provides a video display of the superposed images. An operator then adjusts the relative positions of the component and the substrate to align the bumps and the pads. This may include linear adjustments along perpendicular x-y axes, as well as rotational adjustments. The probe is then withdrawn, and the component is lowered onto the substrate for assembly.
The superposing of images by the dual prism assembly provides a convenient and accurate visual technique for determining alignment. However, in viewing the display of the superposed images, the operator has difficulty in readily distinguishing the features of the component from those of the substrate in order to make the proper adjustment. The operator may then resort to an arbitrary adjustment to one element while observing the shift in the display. This trial-and-error approach increases the correction and prolongs the time for aligning the elements. Moreover, this technique greatly complicates efforts to automate the alignment process. Therefore, there exists a need for an improved precision placement apparatus to permit features of one surface to be readily distinguished from those of the other surface in order to facilitate proper adjustment.